News Corporation, which owns the studio and announced the departure, also reversed a three-year-old decision to consolidate management of its movie and TV operations. Jim Gianopulos, who shared responsibility for those businesses with Mr. Rothman as a fellow chief executive, will now have a narrower focus, running 20th Century Fox Film - the movie studio, the Fox Searchlight specialty label and home entertainment.

The newly separate 20th Century Fox Television will be overseen by its current co-leaders, Dana Walden and Gary Newman. Mr. Gianopulos, Ms. Walden and Mr. Newman will report to Chase Carey, News Corporation's chief operating officer.

Mr. Rothman's resignation takes effect on Jan. 1. In an internal e-mail addressed "dear friends," Mr. Rothman, 57, offered no details about his decision to leave after 18 years. But he may have chafed at News Corporation's decision to strip television from his control. In his e-mail, Mr. Rothman said he needed "some new challenges and to write a new chapter."

Although Fox insiders were stunned, the movie capital has been rife with speculation in recent weeks that Mr. Rothman was a candidate for the top movie job at Universal Studios, where the president, Ron Meyer, is expected to retire.

In his note to the staff, Mr. Rothman - a company man to the degree that he used Fox's signature trumpet fanfare as his cellphone ring tone - spotlighted the role he played in creating Fox Searchlight, which won a best picture Oscar with "Slumdog Millionaire." He also listed his involvement with the two highest-grossing movies in history, "Avatar" and "Titanic," as two of his proudest achievements.

Mr. Rothman is an ardent film lover who hosted his own screenings of classic films on the Fox Movie Channel. But he also focused heavily on populist films that could border on the lowbrow, as with the successful "Alvin and the Chipmunks" series.

Mr. Rothman, one of the longest-tenured studio chiefs in Hollywood, started his career as an entertainment lawyer and worked at Columbia Pictures under David Puttnam in the 1980s.

Known for its tough cost controls, Fox is coming off a difficult stretch. Films like "Prometheus," "Abraham Lincoln: Vampire Hunter" and "We Bought a Zoo" were disappointing at the box office, and Mr. Rothman has been waiting for James Cameron to finish scripts for two "Avatar" sequels that now will not arrive before 2015.

But Mr. Rothman - exhibiting his colorful style - expressed confidence in his internal e-mail over the coming Fox slate, which includes "Life of Pi," an Oscar hopeful directed by Ang Lee. "A bunch of you guys know what the tattoo on my ankle says," he said. "It's what I wish for us all: 'Excelsior!' "

This is a more complete version of the story than the one that appeared in print.